1. Field of the Invention
The present invention generally relates to an image forming apparatus.
2. Description of the Related Art
Conventionally, a temperature rise occurs in an electrophotographic image forming apparatus and its components when a large amount of sheets is processed by the image forming apparatus continuously over an extended period of time; the temperature rise is caused because an image forming unit is kept to be driven over the extended period of time. Known typical examples that take countermeasure against this temperature rise include image forming apparatuses that use a cooling fan, a duct, and/or the like to prevent the temperature inside the image forming apparatus and temperatures of its components from increasing to a certain temperature or higher and image forming apparatuses of high-speed type that include an air conditioner for adjusting the temperature inside the apparatus and perform temperature control.
Known control schemes for such an image forming apparatus, in which the temperature of a fixing roller locally increases when the apparatus continuously processes small-size sheets, include control of directly monitoring the temperature of the fixing roller, thereby temporarily lengthening sheet-feed intervals or making uneven temperature distribution on the fixing roller even.
Disclosed in Japanese Patent Application Laid-open No. 2010-134407 is an image forming apparatus that calculates a developing-motor variable temperature based on an operation mode of the image forming apparatus, estimates a power-supply-off time over which a power supply is cut off based on a change in the temperature of a fixing thermistor, corrects the developing-motor variable temperature based on the power-supply-off time, and calculates an estimated temperature of the developing motor by adding an environmental temperature to the corrected developing-motor variable temperature in order to control the image forming apparatus appropriately in a manner to prevent an excessive temperature rise of the developing motor without directly detecting the developing motor. When the estimated temperature of the developing motor has increased to be equal to or higher than 100° C., this image forming apparatus performs an image forming process intermittently until the temperature decreases to be lower than 80° C. The intermittent image forming process is performed by repeatedly performing a loop of carrying out the image forming process in a continuous manner and entering a standby state where the image forming process is not performed.
Disclosed in Japanese Patent Application Laid-open No. 2006-251504 is an image forming apparatus that counts the number of dots in a toner image formed on an image carrier. When the counted value is equal to or greater than a predetermined reference value, the image forming apparatus lowers the temperature of a toner layer on a developing roller in order to prevent toner from fixing and sticking to a toner regulating member when a large amount of toner is consumed by performing a series of image forming operations after suspending rotation of the developing roller for a predetermined period of time.
However, the conventional image forming apparatus that cools inside the apparatus using a cooling fan and/or a duct is disadvantageous in that limitation of size, structure, layout, or the like of an image forming apparatus body imposes restriction on the degree of temperature that can be reduced inside the image forming apparatus.
Furthermore, there can be a case that the temperature of some portion where a temperature rise matters in the conventional image forming apparatus cannot be monitored and therefore temperature control cannot be performed. Particularly when an image forming unit that includes a developing unit is continuously driven over an extended period of time, there can be a case where temperatures of a sliding portion(s), such as a bearing, and a developer itself inside the developing unit are considerably increased, undesirably causing the developer (toner) to fuse inside the developing unit. Unfortunately, it is difficult to directly monitor the temperature of the sliding portion and the developer itself.
The image forming apparatus disclosed in Japanese Patent Application Laid-open No. 2010-134407 cannot prevent toner fusing caused by an excessive temperature rise of the toner on the developing roller inside the developing unit.
The image forming apparatus disclosed in Japanese Patent Application Laid-open No. 2006-251504 has not solved a problem that the temperatures of the sliding portion(s), such as the bearing, and the developer inside the developing unit are undesirably considerably increased when the number of dots in the toner image is small, causing toner to fuse in the developing unit.
Therefore, there is a need for an image forming apparatus capable of preventing fusing of a developer on a developer carrier caused by an excessive temperature rise without performing computation of estimating the temperature of the developer carrier or the developer in a developing unit and while avoiding a decrease in efficiency during continuous image forming operation.